1. Field of the Invention
The present invention relates to a printed wiring board, in particular, to a printed wiring board that can remove the conduction noise transmitted through a ground.
2. Description of the Related Art
There are many conventional printed wiring boards that include both a digital circuit and an analog circuit on a single printed wiring board. Noise suppression becomes more essential for the devices that are mounted on such printed wiring boards as their operating speed becomes faster. Techniques for noise suppression in these printed wiring boards may include a technique that prevents conduction noise from the ground of a digital circuit from flowing into an analog circuit or a wireless communication circuit by separating the ground of the digital circuit and the ground of the analog circuit or the wireless communication circuit.
For example, Japanese Patent Application Laid-open No. H 09-199818 describes an invention in accordance with such techniques, in which influx of noise generated in a digital circuit can be prevented by constructing a printed inductor as a structure for separating a signal ground and a frame ground or the ground of an analog circuit and the ground of a digital circuit, wherein the printed inductor has a zigzag or spiral wiring pattern to connect these grounds. However, when high-speed signal transmission through a high-speed signal line is used for communication between an analog circuit and a digital circuit, the invention requires a wider pattern width for the printed inductor, when the high-speed signal line pattern is laid out along the shape of the printed inductor so that the return current path necessary for high-speed signal transmission is ensured. Therefore, the inductance value is reduced and the noise generated in the digital circuit may not be effectively suppressed. Additionally, the quality of the signal deteriorates. Moreover, the method disclosed in Japanese Patent Application Laid-open No. H 09-199818 prevents the noise generated in the digital circuit from flowing into the analog circuit by constructing an inductance with a spiral printed inductor as a ground. However, this method also blocks the return signal in the ground directly below the signal line, and thus results in a deteriorated signal. Therefore, the exchange of a digital signal between devices becomes difficult.
Also, Japanese Patent Application Laid-open No. 2006-245081 discloses an invention that provides a digital/analog hybrid printed wiring board with an improved quality of waveform of the signal between the digital circuit and the analog circuit. This is achieved by creating an inner layer wiring pattern between the through holes formed in the digital circuit part and the analog circuit part and creating a spiral return current path including a plurality of through holes formed along the longitudinal direction of the inner layer wiring pattern.
In this invention, it is necessary to increase the pattern width of the spiral conductor to some extent, which constitutes a return current path for the inner layer wiring pattern between the digital circuit and the analog circuit. Therefore, although the shield effect on the wiring pattern can be expected, the inductance value is reduced, and thus, the noise generated in the digital circuit cannot be suppressed effectively.
Consequently, the printed wiring board needs to be redesigned in order to prevent the noise from the digital circuit from flowing into the analog circuit and modification of the design is required.